1. Field of the Invention
The present invention relates to a ceramic-molding binder, especially a compression-molding binder for ceramics, a ceramic-molding composition, and a method for producing a ceramic molding.
2. Description of the Related Art
The properties of ceramics have recently been exploited for use in a broad range of applications such as electronic materials, magnetic materials, optical materials, and refractory materials. Improvements in terms of physical properties, formability, and the like have been demanded in such varying applications. For example, more compact and homogenous products are desired for the sake of better thermal, electrical, mechanical properties and the like. In the fields of mechanical parts, electrical parts, and the like, there is also a need for products with more complex shapes and greater sizes. Meanwhile, the recent miniaturization of electrical products in particular has led to increasing demand for smaller components with more complex shapes than in the past. There has been much research in the areas of ceramic powder starting materials and molding methods to meet such demand.
The use of powders with a fine particle diameter and methods for improving the purity of ceramic powders have been proposed in the past as methods for improving the thermal, electrical, mechanical, and optical properties of final ceramic molded products. In terms of molding, the use of such methods can result in a greater particle surface area and lower plasticity because of impurities in the starting material, and greater amounts of an organic binder are also required.
When forming more complex products of greater size, there is a need for higher strength during the processing or handling of green moldings, which are unsintered moldings. The greater complexity and-smaller size of recent products has also lead to demand for green moldings with greater formability and strength than before. Problems occurring during the handling of green moldings or the formation of smaller moldings with more complex shapes include A) greater molding difficulties and defects in portions of green moldings, B) greater difficulty handling green moldings which are far more brittle, and C) cracks in ceramic moldings after the green moldings have been sintered.
The amount of organic binder that is added has conventionally been increased to deal with such problems. However, increasing the amount of organic binder that is added leads to the following problems with ceramic powder starting materials.
1) Problems Removing the Binder
The binder must be removed before the green moldings are sintered, but the increase in exothermic heat and the amount of decomposing gases at such times due to the large amount of organic binder can result in the danger of cracks caused by explosions or the like, and require a longer time to remove the binder.
2) Problems with Contamination by Impurities, etc.
The addition of greater amounts of organic binder can lead to greater contamination by impurities and more carbon residue after the green molding is sintered, resulting in a final product with lower purity.
3) Problems with Dimensional Precision
Increases in the amount of organic binder also lead to greater shrinkage when green moldings are sintered, and a subsequent loss of dimensional precision.
Although a variety of binders have been studied to resolve such problems, there are very few binders which give moldings with higher strength, density, and homogeneity when added in lower amounts. Conventionally known examples include vinyl alcohol polymers with specific hydrophobic groups and hydrophilic groups (sometimes abbreviated as PVA polymers) (such as in Japanese Examined Patent Application (Kokoku) 63-44709 (Japanese Unexamined Patent Application (Kokai) 59-128265, with family members U.S. Pat. No. 4,492,783 and EP 0116300)), and vinyl alcohol polymers having specific monomer units (such as in Japanese Examined Patent Application (Kokoku) 6-6504 (Japanese Unexamined Patent Application (Kokai) 62-70254, with family members U.S. Pat No. 5,034,451 and EP 0220491), and Japanese Unexamined Patent Application (Kokai) 9-136916). These known binders can be successfully used in small amounts, have good miscibility, and are better than conventional binders in terms of performance such as strength, but cannot be considered to provide satisfactory, performance when forming smaller products of more complex shape, particularly by compression-molding.
An object of the present invention is to provide a ceramic-molding binder which can be added in lower amounts to starting material powder so as to avoid the aforementioned problems 1) through 3), which in particular affords better green molding formability during the molding of smaller products, and which furthermore affords better yields of ceramic moldings, for example, without any of the aforementioned problems A), B), and C), as well as to provide a ceramic-molding composition and a method for producing a ceramic molding.
As a result of extensive research for achieving the aforementioned objectives, the inventors perfected the present invention upon finding that the use, even in lower amounts, of a ceramic-molding binder comprising-mainly a vinyl alcohol polymer having an ethylene unit content of 2 to 19 mole %, a polymerization degree of 200 to 2,000, a degree of saponification of 80 to 99.99 mole %, and a carboxylic acid and lactone ring content of 0.02 to 0.4 mole %, afforded better green formability particularly during the formation of ""smaller products of complex shape in various molding methods, especially compression-molding, that the resulting green molding had better handling properties, and that the subsequently obtained sintered ceramic molding had far fewer cracks, resulting in better yields.
That is, the present invention provides a ceramic-molding binder, comprising a vinyl alcohol polymer having an ethylene unit content of 2 to 19 mole %, a polymerization degree of 200 to 2,000, a degree of saponification of 80 to 99.99 mole %, and a carboxylic acid and lactone ring content of 0.02 to 0.4 mole %.
The present invention also provides a ceramic-molding composition comprising 0.1 to 20 weight parts of the ceramic-molding binder per 100 weight parts ceramic powder.
The present invention furthermore provides a method for producing a ceramic molding, comprising drying an aqueous kneaded mixture of the above ceramic-molding composition to form granules, and molding the granules followed by sintering.
Of binders for various molding processes, the ceramic-molding binder as above is preferably useful as a compression-molding binder for ceramics. Accordingly, the present invention also provides: a compression-molding binder for ceramics, comprising a vinyl alcohol polymer having an ethylene unit content of 2 to 19 mole %, a polymerization degree of 200 to 2,000, a degree of saponification of 80 to 99.99 mole %, and a carboxylic acid and lactone ring content of 0.02 to 0.4 mole %; a ceramic-molding composition comprising 0.1 to 20 weight parts of the compression-molding binder for ceramics per 100 weight parts ceramic powder; and furthermore a method for producing a ceramic molding, comprising drying an aqueous kneaded mixture of the above ceramic-molding composition to form granules, and molding the-granules followed by sintering.